Physical Oceanography

1. Physical Oceanography

2. Outline • Governing Equations (sorry about this) • Geostrophy • Ekman Dynamics • Vorticity • Circulation Patterns & Water Masses • Circulation Theories • Regional Oceanography • Ocean Factsheet

3. Governing Equations • Navier-Stokes The material derivative as modified by Coriolis equals the pressure gradient over the density and some kind of drag/viscosity. • Conservation of Mass

4. Governing Equations • Hydrostatic Equilibrium The change of pressure with depth equals the density times gravity. The negative sign is a convention that applies when z increases downward. • Equation of State Density is modified by thermal expansion and saline contraction.

5. Geostrophy: • Applies when the Rossby radius is small. Ro = U/fL, i.e. when Coriolis is important relative to inertia. • Barotropic flow: isobars and isopycnals are parallel • Baroclinic flow: isobars are inclined relative to isopycnals • Quasi-geostrophy: even when Ro ~1, geosptrophic equations can be useful if vertical mixing is small and you look at timescales > day in a stratified fluid.

6. Ekman Dynamics: Ekman Spiral

7. Ekman Dynamics: Coastal

8. Vorticity: • Potential vorticity = (planetary vorticity + relative vorticity)/height • Potential vorticity is ALWAYS conserved • Planetary vorticity increases poleward • Relative vorticity increases when a) the water column is stretched b) shear from a clockwise flow • Vorticity is positive in the counterclockwise direction

9. Water Masses • North Atlantic Deep Water: forms sporadically in convective cells in N. Atlantic. ~17 Sv • Antarctic Bottom Water: forms in polynyas.-1.9 to .3 deg C, 34.7 ppt. ~17 Sv • Antarctic Intermediate Water: 2-4 deg C, 34.3 ppt

11. Ocean Circulation Patterns

12. Ocean Circulation Theories Sverdrup interior: the curl of the windstress generates flow. In the ocean, the curl is negative, leading to a clockwise circulation.

13. Regional Oceanography: Currents • Gulf Stream: 30 m/s @ Straits of Florida, 70-100 @ Cape Hatteras. Western Boundary Current (WBC) • Brazil current: WBC, 4-20 m/s (WBC) • Kurishio: WBC, 50 m/s (WBC) • Agulhas: 100 m/s, goes around S. Africa. • Antarctic Circumpolar Current: ~100 Sv through the Drake Passage • Indonesian Throughflow: ~10 Sv

14. Regional Oceanography: Equatorial Currents Speeds from 20-50 m/s NEC & SEC flow to the west ECC & EUC flow to the east

15. Regional Oceanography

16. Ocean Factsheet • Pacific Ocean: 52% of ocean area, mean depth 4000m • Atlantic Ocean: 25% of ocean area, mean depth 3300m • Indian Ocean: 20% of ocean area, mean depth 3900m • Ocean volume: 1.4E9 cubic km of water • Salinity: mainly from Cl (55%) and Na (30%), ranges from 33 to 37 ppt, avg. is 34.9 • Temperature ranges from -2 to 30 deg C, avg is 3.5 deg C. • 75% of the ocean has a salinity of 34-35 ppt and a temperature of 0-6 deg C. • Average sea surface temperature: 17.5 deg C.

17. Ocean Circulation 1 What drives ocean circulation? Seawater flows along the horizontal plane and in the vertical. Typical speeds of the horizontal flow or currents are 0.01-1.0 m/s; vertical speeds within the stratified ocean are much smaller, closer to 0.001 m/s. Two forces produce the non-tidal ocean currents: the wind exerting a stress on the sea surface and by buoyancy (heat and freshwater) fluxes between the ocean and atmosphere that alter the density of the surface water. The former induces what we call the wind driven ocean circulation, the latter the thermohaline circulation. The wind driven circulation is by far the more energetic but for the most part resides in the upper kilometer. The sluggish thermohaline circulation reaches in some regions to the sea floor, and is associated with ocean overturning linked the formation and spreading of the major water masses of the global ocean, such as North Atlantic Deep Water and Antarctic Bottom Water. 2 Wind induced upwelling: The wind stress acting on the surface layer of the ocean induces movement of that water. This is called Ekman Layer transport, which extends to the surface 50 to 200 meters. The Ekman transport is directed at 90° to the direction of the wind, to the right of the wind in the northern hemisphere, left of the wind in the southern hemisphere. As the wind varies from place to place, Ekman transport can produce divergence (upwelling) or convergence (sinking) of surface water. 3 Geostrophic Currents: The surface layer is less dense (more buoyant) than the deeper layers, therefore a spatially variable Ekman transport field acts to redistribute the buoyant surface water: thinning the buoyancy surface layer in divergence regions, thickening the buoyant surface layer in convergence regions. As the ocean is in hydrostatic equilibrium, the redistribution of the buoyant surface layer induces sea level "valleys" in divergent regions and "hills" in convergence regions. While these hills and valleys amount to only a 1.5 meter in amplitude, they are sufficient to induce horizontal pressure gradients which initiate the wind driven circulation following the geostrophic balance concept. The ocean currents are for the most part geostrophic, meaning that the Coriolis Force balances the horizontal pressure gradients. The wind driven circulation is characterized by large clock-wise and counter clock-wise flowing gyres, such as the subtropical and sub polar gyres. The Antarctic Circumpolar Current is also a wind driven current; in contrast to the subtropical gyres it reaches the sea floor. 4 Thermohaline Circulation: As surface water is made denser through the removal of heat or freshwater, the surface layer descends to deeper depths. If the stratification is weak and the buoyancy removal sufficient, the descent would reach the deep sea floor. Such deep reaching convention occurs in the northern North Atlantic (North Atlantic Deep Water) and around Antarctica (Antarctic Bottom Water). The thermohaline circulation engages the full volume of the ocean into the climate system, by allowing all of the ocean water to 'meet' and interact directly the atmosphere (on a time scale of 100-1000 years).